The invention relates to a process for preparing a mixture used for the production of electrodes, for the production of aluminum in particular, comprising dry material and an electrode binding agent, the said preparation involving mixing, degassing and a temperature change.
Basically there are two well known processes for preparing the mixture required for the production of electrodes:
1. Pre-crushed residual pieces from used electrodes are sieved and stored as coarse grain material in batteries of silos. Petrol coke is combined with the fines from that crushed material to give a medium sized particulate material. The oversized material from this stage is ground and recirculated, the undersized material and the outsize material from the medium size fraction is ground to dust in a ball mill. Coarse grained residual material, the combined coke-medium size and dusts are led to batch type scales and combined in chosen amounts with green rest material. In double arm kneaders the dry constituent and the green rest are heated and mixed with solid or liquid pitch. This mass is transported to a shaping device.
2. A further process is such that petrol coke and pre-crushed rest from used electrodes are taken from silos, mixed, dried, crushed and divided with sieving devices into coarse, medium and fine fractions. Coarse and fine grinding mills grind the outsize material from the coarse and medium fraction silos. Subsized and possibly also the outsized material from the medium and fine material silos are ground to dust in a ball mill. After the sizing silos the dry component is fed, via continuous weighing facilities, to a pre-heating facility and continuous kneading facility where the green rest and solid or liquid pitch are added, and the mixture then transported to an electrode shaping or forming facility.
With respect to the components of the mixture, the existing systems can be divided into those which operate:
discontinuously (e.g. batch mixing units of the double arm kneader type), or
continuously (e.g. extruder or cokneader).
The known processes feature considerable disadvantages, in particular with respect to
(a) the mixing process,
(b) the cooling of the mixture,
(c) work-place hygiene and environmental problems.
The disadvantages concerning the mixing process depend on the device used for this.
For example in the case of double arm kneaders it is difficult to overcome wear problems because of design features, which lead to large maintenance costs. The normal bottom emptying of modern double arm kneaders tends to cause sticking and therefore less efficient use of the unit as well as problems with work-place hygiene. The emission of carbon dust, steam and pitch fumes which occurs with double arm kneaders is also difficult to overcome.
Extruders or coextruders involve high investment costs. Due to the strong mixing effect pronounced wear occurs on the shaft and housing. Overcoming this incurs high maintenance costs. Also the throughput can be altered only very slightly.
If it is found necessary to employ a further kneader for higher production, due to the continuous working nature of this unit it is also necessary to install a whole series of other facilities such as storage silos, weighing facilities and pre-heating units.
Furthermore, the effects of the various apparent densities and particulate strength of the various cokes on the density and strength properties of the anodes is very marked under constant mixing conditions.
The cooling of the mixture presents great difficulties in all methods of electrode manufacture. Here it is the wetting properties of the raw materials which determine the mixing temperature, which lies between 150.degree. and 170.degree. C.
The maximum temperature for forming the electrodes on the other hand is limited by deformation and cracking of the electrode. The lower temperature for this is limited by insufficient density, strength and resistance, so that for extruded electrodes the production temperature lies between 90.degree. and 120.degree. C., in the case of vibration-settled electrodes between 130.degree. and 150.degree. C.
In order to keep the mixing and forming temperature within a narrow range, the green mass must be cooled in a specific manner. Using the methods employed up to now, however, there are a number of disadvantages associated with this cooling viz.,
(a) The emission of pitch fumes leads to pollution of the working area and the surrounding environment.
(b) The flow properties and the thermal conductivity of the electrode mass lead to the formation of lumps and therefore inhomogeneities in the final electrode, strength problems and cracking.
(c) Temperature control is difficult because of measurement and control problems.
(d) Control mechanisms and operating parameters are not clearly defined.
The disadvantages involving work-place hygiene and environmental problems are closely related to the methods used up to now for cooling the mass. For example, when cooling 15 tons of mixture per hour from 150.degree. C. to 110.degree. C. by the most widely used method of direct cooling with air, about 30,000 m.sup.3 of air are heated by 20.degree. C. and about 4 kg of condensed tar fumes are given off per hour. The air used for cooling is then enriched with tar fumes and has to be purified, which is possible only at great expense. The present high standards required by environmental regulations mean that this is a disadvantage which is very favorable for existing units.
The object of the present invention is therefore to develop a process for preparing a mixture of dry material and binding agents for the production of electrodes, in particular electrodes for the production of aluminum by mixing, degassing and changing the temperature such that these disadvantages are overcome and also such that the said process is economically favourable for the production of electrodes.